New plant hardiness maps show that winter is warming, but gardeners shouldn’t get carried away
Published: 11-27-2023 4:55 PM
Modified: 11-29-2023 8:58 AM
On Nov. 11, the gorgeous ginkgo tree on the Durham campus of UNH dropped all of its bright yellow leaves at once, an eye-popping move that ginkgo trees are famous for doing at the season’s first hard frost.
The annual leaf-drop has long been cause for minor celebration in Durham but a look at records since 1977 by UNH professor Serita Frey showed a not-very-celebratory fact: This was the latest leaf-drop on record.
It’s just another sign that our winters are getting warmer and shorter.
Here’s another sign of changing winters: the new plant hardiness maps from the U.S. Department of Agriculture, released earlier this month.
The maps, used by many gardeners to decide what’s viable to put in their yards and raised beds, are based on the expected minimum temperature during the winter – that is, the hardest freeze you’re likely to see, judging from three decades of recent winters.
The maps are based on 30 years of records; the new map is based on averages from 1991 to 2020, whereas the old map used averages from 1975 to 2015. Winters warmed up a bit over that 15-year move.
For New Hampshire, the big change is in the south.
The new maps say that all of Rockingham County and most of Hillsborough County have shifted from Zone 5b, where plants should be prepared for the mercury to hit 15 below at least once a winter, to Zone 6a, where the mercury is unlikely to get below minus-10. (Higher Zone numbers mean warmer winters.)
In the old map, Zone 6a didn’t get closer to Concord than Salem and Exeter but in the new map it sneaks up the Merrimack River valley clear into Bow.
It’s a definite indication that winters are getting less wintery, but what does this mean for area gardeners? Not as much as you might think.
“We’re seeing on average warmer winters, and that’s what this map directly relates to. … But it’s not going to be a huge effect,” said Steph Sosinski, program manager for home horticulture for UNH Extension. Planting anything at the edge of a zone still risks it being clobbered by a cold snap.
“This past winter was a great example. We have plants that are typically adapted to Zone 5 but we had that deep, deep cold snap in February that still damaged the plants,” she said.
Lisa Mills, owner of Nicole’s Greenhouse and Florist in Pembroke, thinks the USDA has always underplayed the severity of our winters for plants and thinks the new map just makes it worse.
“There’s no way in God’s green earth that Bow’s a Zone 6!” she said.
A big issue, she said, is a property’s micro-climate. What works in one part of your yard may not work elsewhere.
“If you’re planting it all by itself on the top of a hill, why bother? It’s like a magnet waiting for wind,” she said.
“I do treat everything we get here as a Zone 4,” she said because all it takes is one or two unexpected freezes to do damage. By USDA standards, Zone 4 doesn’t start until you get past Squam Lake.
She pointed to plants like hibiscus and rose of Sharon, which are theoretically Zone 5 and fine for Pembroke. “When my customers buy those I tell them, you need to give them added protection; if you mulch 3 inches regularly (for winter), this needs 6 and wind protection.”
Mills has a number of rules of thumb from years of observation – the Canterbury exit on I-93 is the line where snow gets more predictable; if you want your rosemary and lavender to come next year it helps to be south of the Hooksett tolls.
Even so, the new USDA map shows that on average our winters are looking a little less like a Currier & Ives print every year. And that’s going to keep happening for years to come, no matter what plant hardiness zone you’re in.